added Porter stemmer test cases (from http://snowball.tartarus.org/algorithms/porter/diffs.txt)
completed tests for Porter helpers
added simple Porter test cases and test routine for the major set
corrected fundamental aspects of Porter algorithm, which can now return something for every input in the Snowball test set (but not always the correct thing)
fixed Porter stemmer so that it matches longest suffix and then quits (i.e. the stem of 'argument' is 'argument')
This now passes the whole Snowball test set
updated Porter algorithm & Snowball test set to match the "Points of difference from the published algorithm"